As of kde-4.7.90+, kmix is starting to chew up both cpu and memory after some time. A quick-n-dirty workaround, is to simply quit/restart kmix.
The work around sadly is not permanent, I have since found another related report on kde.bugs https://bugs.kde.org/show_bug.cgi?id=288675
needs testing, but upstream fixes look promising... %changelog * Fri Dec 30 2011 Rex Dieter <rdieter> 6:4.7.95-2 - kmix chewing up cpu and memory (#766678,kde#288675)